Electroacupuncture alleviates inflammatory pain via the activation of GABAergic inhibitory interneurons in                                                    mice spinal dorsal horn

Background

Although the analgesic effect of electro-acupuncture (EA) has been suggested by large amounts of clinical and pre-clinical evidence, the fundamental neural mechanisms underlying EA-mediated alleviation of pain hypersensitivity remain poorly understood. The spinal dorsal horn (SDH) serves as the primary center for the integration of nociceptive inputs, and SDH neuronal dysfunction contributes to the development and maintenance of pathological pain. In the present study, we aimed to investigate the role of superficial SDH neurons recruited in EA’s analgesic effect.

Materials and methods

C57BL/6 mice (20 - 30 g) were randomly divided into control (vehicle of the complete Freund’s adjuvant (CFA)), CFA, CFA+EA, and CFA+sham EA groups. CFA was injected into the dorsal surface of the left hindpaw (20 µl) for CFA-induced inflammatory pain. Mice of the CFA+EA group received EA administration on the left Huantiao  (GB30) and Yanglingquan (GB34) once every other day, starting from day 1 to day 8 after CFA injection. EA (1 mA and 0.3 ms) was administered at the frequency of 2 Hz for 30 min. The tactile withdrawal threshold of mice was measured by using the “up-down” method.Immunohistochemistry was performed to observe the expression of C-fos, GABA, and Calretinin (CR) in superficial SDH.  

Results

CFA-injected mice rapidly and persistently presented mechanical hypersensitivity under the von Frey test. Mice in the CFA+EA group displayed a significantly raised mechanical withdrawal threshold compared to mice in the CFA group (Fig. A). Double immunostaining showed that CFA treatment induced C-fos protein expression in CR+ neurons in the ipsilateral superficial SDH compared to control mice, while EA intervention markedly decreased the percentage of C-fos/CR double-labeled SDH neurons   (Fig. B). In contrast, EA treatment resulted in increased colocalization of GABAergic inhibitory interneurons with C-fos in SDH (Fig. C).